Multistage polymerization processes have been used to make blends of two or more polymer components known as reactor polymer blends. See, for example, U.S. Pat. Nos. 4,740,550 and 4,740,551. A typical multistage polymerization process comprises: 1) in a first stage, polymerizing monomers with an active catalyst to polymerize a first polymer component; and 2) transferring the contents of the first stage into a second stage where the active catalyst from the first stage is used to make a second polymer component. See, for example, U.S. Pat. Nos. 5,280,074, 5,322,902 and 5,684,097 where a crystalline polyolefin component is prepared in liquid monomer in a first bulk slurry stage and a copolymer is prepared in a second gas phase stage. Typically the catalyst composition employed comprises one catalyst throughout the process and often this catalyst is particularly suited for making one of the polymer components but less suitable for making the other polymer component(s).
An alternative solution to this problem is to add a catalyst in the second polymerization which is different from the catalyst used in the first polymerization stage. A catalyst advantageous for making the first polymer component is added during the first polymerization stage and a second catalyst advantageous for making the second polymer component is added during the second stage. See, for example, U.S. Pat. No. 5,648,422. It addresses the use of a catalyst particularly suitable for the preparation of crystalline polypropylene is a first stage, or stages, with introduction of a second catalyst particularly suitable for making a copolymer selected for the final blend. The first transition metal catalyst may be deactivated, and the second is added with activator, typically a metallocene pre-contacted with an alumoxane activator. A drawback to this solution is the cumbersome step of adding an additional catalyst and activator and effectively having the second catalyst uniformly dispersed within the first polymer component such that the second copolymer component is prepared with an essentially uniform dispersal within the first polymer component.
Another solution to this problem is to use a mixed catalyst composition comprising two catalysts used throughout each polymerization stage. One of the two catalysts is advantageous for making a first polymer component of a reactor blend while the second catalyst is advantageous for making a second polymer component. See, for example, WO 98/10016. Other relevant art includes U.S. Pat. No. 5,714,427. The drawback of such a solution is that both catalysts are active throughout any given stage of the polymerization process and the particular advantages of the two for the different polymer components can be diminished. Efforts have been made to selectively deactivate one of the catalysts but such a step is cumbersome and not preferable.
The above methods can be undesirable because the tailoring of final reactor blends is limited as described above. It is thus desirable to design a process which is particularly suitable both for making a first polymer component of a reactor blend and for making at least one other polymer component of a reactor blend.